1. Field of the Invention
The present invention relates generally to wireless networks, and more particularly to scanning and establishing communications in a wireless local area network.
2. Related Art
In order for a mobile station to determine which network to join, the mobile station must first scan for available networks. Scanning can be passive or active. Passive scanning involves listening for beacon frames from APs (access points). Active scanning, on the other hand, involves the transmission of Probe Request frames for soliciting a Probe Response frame from access points in the area. Receiving a beacon frame (passive scanning) or a Probe Response frame (active scanning) from an access point allows the mobile station to learn the availability and/or the characteristics of the network that is coordinated by the access point. Scanning, either passive or active, can take place on one or more radio channels according to the number of radio channels available for the wireless network.
One of the significant problems of the conventional scanning methods is that they are not fast enough to prevent communication interruption during handoff. In case of IEEE 802.11 Wireless LAN, an empirical analysis of handoff process found that active scanning may have high latency of as much as 50 msec per channel. Details can be found in IEEE 802 Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, IEEE Standard 802.11, 1999 and “An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process,” by Arunesh Mishra, Minho Shin, William Arbaugh, submitted to ACM CCR, both of which are incorporated by reference in their entirety. Passive scanning in IEEE 802.11 has an even higher latency since the mobile station must stay on each channel for at least one beacon interval, whose recommended value is 100 msec. A more detailed description can be found in Matthew S. Gast, “802.11 Wireless Networks—The Definitive Guide,” O'Reilly, April 2002, which is incorporated by reference in its entirety. Large scanning time may contribute to TCP timeout, triggering TCP congestion avoidance algorithm, which results in decreased throughput. The large scanning time may also seriously deteriorate quality of voice service, since the maximum interruption time allowed for voice application is 50 msec (ideally, interruption time should be kept less than 35 msec).
This situation will be more aggravated if PCF (Point Coordination Function) is used in the APs to be scanned. If an AP is operating PCF, only an associated mobile station has the right to be added to the polling list, and during a CFP (Contention Free Period) only the mobile station in the polling list may access the wireless medium if polled by the AP. Since a mobile station in active scanning is not associated with the APs to be scanned, the mobile station cannot send Probe Request frames during the CFP. As a result, scanning latency is dramatically increased.
Therefore, a scanning method for wireless networks that has smaller latency is desired.